I did a speed test on my new 48X CD-RW drive. I was reading a standard CD-R disc with data on it. I got an observed average speed of 23X. However, the graph had two lines on it, one green the other yellow. The green line was in the 48X range and the yellow one ranged from 20X to 28X and obviously corresponded to the average speed result. Why are there two lines on the graph and what do my results mean?